This invention relates generally to gas turbine engine variable stator vane assemblies and, more particularly, to wear resistant coatings used within the variable stator vane assembly.
In a gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The compressor is made up of several rows or stages of compressor stator vanes and corresponding rows or stages of compressor rotor blades there between. The stator vane rows are situated between the rotor blade rows and direct airflow toward downstream rotor blades on the rotor blade row. After leaving the compressor, the air/fuel mixture is combusted, and the resulting hot combustion gases are passed through the turbine section of the engine. The flow of hot combustion gases turn the turbine by contacting an airfoil portion of the turbine blade, which in turn rotates the shaft and provides power to the compressor. The hot exhaust gases exit from the rear of the engine, driving the engine forward. Optionally, a bypass fan driven by a shaft extending from the turbine section, which forces air around the center core of the engine and provides additional thrust to the engine.
To increase the operating capacity of the compressor, at least some of the compressor stator vane rows are designed with vanes that can rotate in around an axis that is in its longitudinal direction to adjust the angular orientation of the vane with respect to the airflow traveling through the compressor. The adjustment of the angular orientation allows control of the amount of air flowing through the compressor. Variable stator vane designs typically allow for about 45° rotation of the stator vane to optimize compressor performance over the operating envelope of a gas turbine engine. The variable stator vane structures include an outer trunnion disposed in a complementary mounting boss in the stator casing for allowing rotation of the vane relative to the casing. A lever arm is fixedly joined to a coaxial stem extending outwardly from the vane trunnion. The distal end of the lever arm is operatively joined to an actuation ring that controls the angle of the vane. All of the vane lever arms in a single row are joined to a common actuation ring for ensuring that all of the variable vanes are positioned relative to the airflow in the compressor stage at the same angular orientation.
A known variable stator vane assembly includes a bushing and washer disposed between a trunnion attached to a variable vane and a casing. The bushing and washer decrease the coefficient of friction between the trunnion and the casing and facilitate rotation of the vane, through the trunnion. The bushing and washer also help prevent wear of the trunnion and casing. A shroud may also be place between the trunnion and casing to prevent wear.
A number of structures in the gas turbine engine, including the bushing and washer structures, used with variable stator vanes are subjected to conditions of wear at temperatures ranging from low temperatures to highly elevated temperatures. In addition, the bushing and washers are subject to high altitude atmospheres. In addition to low temperatures, high altitude atmosphere includes little or no water vapor. Water vapor is required for conventional graphite containing lubricants to maintain lubricity.
Wear occurs when contacting surfaces of two components rub against each other. Typical results from wear include scoring of one or both surfaces, and/or material removal from one or both surfaces. In the bushing and washer system of the variable stator vane assembly, scoring may occur on one or both of the surface trunnion and the casing, both of which are expensive to repair and/or replace. As the surfaces are damaged, they become even more susceptible to the effects of wear as their effective coefficients of friction rise and wear debris is trapped between the wearing surfaces, so that the wear damage accelerates with increasing time in service. Wear debris may include material removed from the wearing surfaces due to wear, or may include foreign particles, such as dust or debris from the air traveling through the engine.
The wear conditions sometimes arise because it is not desirable or possible to firmly affix the two components together to prevent the rubbing action, because of the functionality of the components. An example is a cylindrical bushing used to support a variable stator vane in the compressor section of the gas turbine engine where the element inserted into the bushing (e.g., the vane trunnion) rotates or slides in contact with the surface of the bushing.
When a bushing and washer system fails due to excessive wear, serious problems for the gas turbine engine compressor may occur. The failure of the bushing and washer may create an increase in leakage of compressed air from the interior of the compressor through the variable stator vane assembly, which results in performance loss for the compressor. In addition, failure of the bushing and washer can result in contact between the stator vane and the casing, which causes wear and increases overhaul costs of the engine.
One known material for fabrication of bushing for variable stator vane assemblies is a specially developed composite of carbon fiber reinforcing rods in a polyamide resin matrix manufactured by E. I. Du Pont De Nemours and Company of Wilmington, Del. The bushings are commonly known as VESPEL®CP™ bushings. VESPEL® and CP™ are trademarks that are owned by E. I. Du Pont De Nemours and Company. The polyamide resin used in the VESPEL®CP™ bushings is commonly known as NR150™. The NR150™ trademark is owned by Cytec Technology Group of Wilmington, Del. Although the VESPEL®CP™ bushings have an extended life at temperatures 450–500° F. (232–260° C.), the VESPEL®CP™ bushing have an upper temperature limit of 600° F. (316° C.). Extended operation at temperatures at or above 600° F. (316° C.) limit their operational life. The polymer bushings do not withstand the combinations of high temperature and vibrational loading experienced in the operation of the gas turbine engine well, leading to a relatively short part life.
Another known method for reducing wear on the variable stator vane assembly is placing a carbon-containing antifriction coating on a surface in the variable stator vane assembly. This antifriction coating is a coating fabricated from a material that reduces the coefficient of friction between the surface of the trunnion and the surface of the casing. One carbon-containing component known for antifriction coating is graphite. However, graphite has the disadvantage that water vapor is required to maintain lubricity. Atmospheres at aircraft cruise altitudes do not have enough water vapor present for graphite to be lubricious. Graphite also has the disadvantage that graphite has poor tribological properties in applications that require reciprocating motion. An additional disadvantage of graphite is that graphite begins to oxidize rapidly at temperatures at or greater than 500° C. (932° F.). Some variable stator vane systems may experience temperatures in excess of 500° C. (932° F.). Therefore, a replacement material for graphite in antifriction coating is needed.
Attempts have also been made to coat the stator vane trunnion with a single wear coating. The single wear coating attempts to incorporate the low coefficient of friction materials known in the art with hard, smooth wear resistant coating materials into a single coating on the vane trunnion. However, the single wear coating lacks the ability to maintain the properties of each of the individual components (i.e., fails to maintain both low coefficient of friction and wear resistance). In other words, the single wear coating does not provide all of the desired tribological properties (e.g., reduce wear and low coefficient of friction) required for extended operation of variable stator vanes subject to conditions of high temperature, vibration and high altitude atmospheres.
There is accordingly a need for an improved approach to the protection of gas turbine components, such as variable vane trunnion surfaces, variable vane casing surface or other surfaces in the gas turbine engine against the damage caused by wear. The present invention fulfills this need, and further provides related advantages.